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You need to use the sine rule.

If the three angles are A, B and C and the sides opposite them are named a, b and c then, by the sine rule,

a/sin(A) = b/sin(b) = c/sin(C)

Therefore

b = a*sin(B)/sin(A) = a*y where y = sin(B)/sin(A) can be calculated

and

c = a*sin(C)/sin(A) = a*z where z = sin(C)/sin(A) can be calculated.

then perimeter = p = a + b + c = a + ay + az = a*(1 + y + z)

therefore a = p/(1 + y + z)

or a = p/[1 + sin(B)/sin(A) + sin(C)/sin(A)].

Everything on the right hand side is known and so a can be calculated.

Once that has been done, b = a*y and c = a*z.

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Q: How do you get the sides of the triangle when 3 angles are given and its perimeter?

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One can't. The angle between those is needed.

If you have the length of each of the three sides of a triangle, you can find the perimeter of (the distance around) the triangle by adding the length of the sides. Their sum will be the perimeter of this geometric shape.

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The perimeter of a triangle is the sum of its 3 sides

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